PROJECT SUMMARY The prognosis for pancreatic cancer (PC) patients is poor. Data from several groups has shown that significant infiltration of PC by macrophages decreases the efficacy of chemo-, radiation- and immunotherapy in an animal models and correlates with poor clinical outcomes in patients. The classical assumption has been that these tumor-associated macrophages are derived from circulating monocytes. Recent changes in the field of developmental biology suggest this assumption is not entirely correct and offer a radically new view of macrophage origins in many tissues. It is apparent that tissue-resident macrophages can also arise from embryonic precursors that seed tissues in pre- or perinatal periods. In our recently published study we established a key role for embryonically-derived macrophages (eMACs) in PC progression. We demonstrated that: 1) eMACs expand exponentially during PC progression by in situ proliferation, 2) eMACs are more potent drivers of PC progression than their monocyte-derived counterparts, and 3) eMACs have a distinct tissue remodeling phenotype that significantly enhances PDAC fibrosis in vivo. Thus, a further study of the interactions between various origin-based subsets of macrophages in PC may lead to an understanding of the recalcitrant nature of the disease. Our overall hypothesis is that epigenetically poised, embryonically derived pancreas-resident macrophages are critical regulators of pancreatic fibrosis and early disease progression in PC. To test this hypothesis we will: Aim 1. Determine the mechanisms by which eMACs drive fibrosis and early PDAC pathogenesis. Aim 2. Determine the origin-specific epigenetic drivers of eMAC pro-fibrotic and pro-tumor activity. Aim 3. Determine the impact of eMACs on therapeutic responsiveness. Impact: Our classical assumption was that all TAMs are derived from monocytes, however this may not be true. The fact that embryonic- and/or tissue resident-derived TAMs might impact PDAC progression and response to therapy has significant implications for both basic science and clinical care.